A line of 18-foot-high steel posts spaced four inches apart flank the entrance of part of one of the most biodiverse ecosystems in the United States, and one of the most endangered. Bifurcated by the fence is the Nature Conservancy’s Southmost Preserve near Brownsvillle, Tex., whose threatened species include the Southern yellow bat, the Texas tortoise and the ocelot, an endangered cat whose estimated American population is under 50. One of the few remaining stands of native sable palms in the United States grow there as well.

The posts are part of a 70-mile “pedestrian barrier” between Falcon Dam and the Gulf of Mexico that was built to deter illegal immigration and drug trafficking. It lies anywhere from hundreds of yards to several miles north of the border. Before construction started in 2009, experts expressed concern about the effects of the fence on so-called wildlife corridors in the Rio Grande Valley. Now they are taking stock of the impacts.

“All wildlife roam along corridors,” said Laura Huffman, director of the Texas branch of the Nature Conservancy. “These are nature’s highways. Any time you put an obstacle in a highway, it’s going to affect mobility, the ability of animals to move back and forth.”
At the Southmost Preserve, the bottom of the fence is pierced by 8-by-11-inch openings every 500 feet.These aren’t large enough for many animals, biologists say, nor were they positioned on the basis of existing data on wildlife corridors.

“Smaller animals – young coyotes, weasels, jackrabbits – can get through the holes, but larger animals can’t,” Ms. Huffman said. “The wall seems to have caused changes to movement patterns of many wildlife. We’re seeing tracks of deer and javelina where we weren’t before. I suspect they are having to follow along the fence to attempt crossings.”

The federal Fish and Wildlife Service has used cameras and collars to monitor bobcats in the Rio Grande Valley for more than 12 years. So far it has no evidence that bobcats are using the openings.

Ms. Huffman said the wall also seems to have changed some animals’ movement patterns. “We’re seeing tracks of deer and javelina where we weren’t before,” she said. “I suspect they are having to follow along the fence to attempt crossings.”

For the ocelot and the jaguarundi, another small cat, interbreeding between populations on both sides of the Rio Grande river is considered ciritical to maintaining genetic diversity.

“I don’t think the openings are effective — nobody does,” said Kelly McDowell, the Fish and Wildlife Service’s refuge supervisor for Oklahoma and Texas. She said it was unlikely that ocelots would even find them. “This is a heavy forest-dwelling species that stays in cover, and you’re talking about an area that is cleared out with roads on both sides,” she said. “As a wildlife biologist, I can’t say there is any likelihood that the openings will maintain the connectivity that we think a species like ocelot needs.”

When the barrier went up, Maxwell Pons, the Southmost Preserve’s manager, sprayed fox urine around the openings to help animals find them. But not all species would pick up on the scent, she said.

The barrier also bisects the Anacua Unit of the Texas Parks and Wildlife department’s Las Palomas Wildlife Management Area, isolating 48 acres on its south side. “The fence limits the travel of larger animals,” said the department’s regional director, Len Polasek. “It also prevents animals from getting to the river for water.”

When construction got under way, the department’s biologists identified species likely to be affected by the barrier; among them were 10 plants and animals on federal and state endangered lists, 23 on the state’s threatened list, and dozens of species of concern.

“The border fence and roads associated with it reduce ability of animals to move on the landscape,” said Jon Beckmann, a scientist with the Wildlife Conservation Society. “That isolates populations, which reduces genetic variation.”

Some sections of barrier in the Lower Rio Grand Valley are solid concrete walls intended to retain floodwaters, increasing the possibility that wildlife could be trapped in high water, environmentalists also point out. Even the post-style barrier can worsen flooding by trapping debris.

Mr. Beckmann said that many animals would be deterred from crossing anyway by the increased lighting and the vehicles of border guards and others traveling along the line of posts.

Mr. Pons and Ms. Huffman also worry what will happen to the Southmost Preserve’s native plant nursery, which has provided 70,000 seedlings of 16 trees and scrub to help restore native landscapes. “A lot of people had worked hard and put together a pretty remarkable mosaic of lands protected by state, federal and local agencies and organizations,” Ms. Huffman said.

Several studies support environmentalists’ arguments about the potentially destructive effect of the barriers. A 2009 study in the journal Conservation Biology suggested that the barrier would disrupt movement and distribution of ferruginous pygmy-owls, bighorn sheep and similar species. Another by scientists from the University of Texas and Yale identified three regions where wildlife was most at risk from pedestrian barriers: coastal California, Arizona’s Madrean Sky Island Archipelago and the Rio Grande Valley. (The barriers, mandated by the Secure Fence Act of 2006, extend intermittently across 650 miles of the border, from California to Texas.)

Still, Mr. Beckmann of the Wildlife Conservation Society says he remains optimistic that a solution will be found as the impacts are further documented. “If we get the right constituents together, we can address national security concerns and maintain functioning ecosystems at the same time,” he said. “It can be done. It’s just whether we as a nation decide it is something we want to do.”

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